CLASSES OF PLANTS 75 



1 80. The lowest fungi are the bacteria, a large and impor- 

 tant group, though made up of the simplest and minutest of 

 all organisms. The bacteria are minute unicellular or fila- 

 mentous organisms, so simple in structure that the cell con- 

 stituting an individual seems to be devoid of even the nucleus. 

 To this group belong the germs of many diseases, and the active 

 agents in various processes, such as putrefaction and decay, 

 souring of milk, acid and vinous fermentations, etc. 



181. Cryptogams reproduce by means of spores instead of 

 by seeds. Spores are single cells specially set apart by the plant 

 for the purpose of reproduction. In some cases they are formed 

 by the union of two elements, as in the process of fertilization 

 in Spermatophytes. Another kind of spore is formed merely 

 by the separation from some part of the parent plant of a 

 single cell, which has the power of developing a new plant with- 

 out fertilization. Some of the lowest, simplest Cryptogams, 

 consisting of a single cell, multiply merely by the division of the 

 cell into equal halves (fission). 



Ecology 



182. In our study of the development, form, structure and 

 life processes of a plant we have confined our attention almost 

 entirely to the kinds of plants with which we have been most 

 familiar, i. e., such as grow in soils that are at least moderately 

 productive to the agriculturist and in climates which are the 

 most habitable to man, neither extremely cold nor hot, nor ex- 

 tremely wet or dry. And besides, we have limited our study 

 to the independent, chlorophyll-bearing plants. In these we 

 have seen with regard to every feature of the plant's organiza- 

 tion a remarkable adjustment to its external conditions of 

 existence, or, in other words, adaptation to environment. This 

 has been so apparent at every turn that one might well regard 

 it as a law of nature. However, if there be such a law, it must 



